This invention relates to a plano-milling machine of the closed planer type, which is to say a machine tool in which heavy workpieces, e.g. of up to four tons in weight, are moved horizontally through a bridge structure comprising columns on either sie of the table on which the workpieces are moved spanned above the table by a cross-rail carrying the machining head. The head, which has a spindle to which different tools can be fitted, is movable both perpendicularly (the Z axis) and horizontally (the Y axis) at right angles to the table supporting the workpiece to machine the upper surface of the latter.
At present the heads of such machines comprise a single spindle which is at all times vertical, i.e. rotates the associated collet about a vertical (Z) axis whatever the position of the head. Consequently after machining one face of a workpiece it is necessary to reload it in a new, fixed position on the table to machine another face. This difficult and time-wasting operation may have to be carried out four times after machining the top surface of the workpiece in order to present its side and end surfaces to the milling or other tool engaged in the collet.